The present invention relates to a retractable drill bit system particularly, although not exclusively, for use in oil and gas drilling.
In most forms of ground drilling a drill bit is attached to a lower end of a drill string and the drill string rotated at the ground level to drill a hole in the ground. To increase the depth of the hole being drilled, drill rods are sequentially and individually screwed onto the ground end of the drill string.
There are obvious commercial and technical advantages in being able to change the drill bit when necessary without the need to pull the drill string from the ground. The present applicant has been particularly innovative in the design of a retractable drill bit system for core or diamond drilling. Such a system is described in the applicant's International Application No PCT/AU94/00322 (WO 94/29567). In that system, the core drill bit is segmented into separate fingers and transported to and from the end of the ground drill by use of a running tool. The tool also operates an internal bit locking sleeve that can slide up and down a drive sub attached to the end of the drill string for locking the fingers in place and subsequently releasing them to collapse back on to the tool for retrieval. Once the fingers have been locked in place the tool is removed and a core barrel lowered in to the drill string in the conventional manner and the drill operated to cut a core.
This system has proved to be very successful. Nevertheless, the system is largely limited to drilling applications where the diameter of the drill pipe is similar (marginally smaller) than the diameter of the hole being drilled by the drill bit at the end of the drill pipe. Also that system does not totally eliminate time lost to change the bit, as tripping the running tool for retrieving and installing the bit will consume valuable drill time. It is these limitations that have lead to the development of the present invention.